Automatic gain control system for hyperbolic navigation receivers



FRANTZ W. B. AUTOMATIC GAIN CONTROL SYSTEM FOR HYPERBOLIC NAVIGATION RECEIVERS Filed Jan. 13, 1954 ATTORNEY Dec. 27, 1955 law United States Patent C AUTOMATIC GAIN CONTROL SYSTEM FOR HYPERBOLIC NAVIGATION RECEIV ERS Wilbert P. Long Beach, N. Y., assignor to Sperry vRand Corporation, a corporation of Delaware The present inventionrelates to automatic gain control systems, and in particular to improved automatic gain control systems useful in loran receiver-indicators- In my Patent 2,651,033, l have taught a system of automatic gain control for a loran receiver-indicator in which the gain of the receiver is automatically determined by the strength of the received master pulses.v This AGC system operates in conjunction with an automatic amplitude balance control system to maintain the amplitudes of received output master and slave pulses at a suitably constant and equal peak value.

`In 4the prior art manually operated loran receiverindicators, an operator maintains the amplitudes of receivecl output master and slave pulses at a suitably constant and equal peak value by manipulating a receiver manual gain control and a manual amplitude balance control. The receivers manual gain control is tirst adjusted byA the operator to a position which will allow the loran receiver to amplify the weaker or smaller of the received master or slave pulses to a suitable output level, and then the manual amplitude balance control is adjusted to reduce the amplification of the stronger or larger of the received master or slave pulses until the received output master and slave` pulses are substantially equal in peak Value.. Y

An improvement in these prior art manually operated loran receiver-indicators can be made by reducing the number of manual adjustments, and by providing automatic controls whenever possible as has been taught in my aforesaid patent. The present invention discloses an alternative system of automatic gain control which may be used lin these prior art loran receiver-indicators to replacethe manual control of receiver gain by anv operator, and, this alternative system may be used either with or without'the automatic balance control system as taught in'my'aforesaid patent. The present invention distinguishes over the AGC system ofthe aforesaid patent in that the loran receiver gain is not solely determined by tlie"amplitude of the received master pulses, but is determined'by the amplitude of the smaller of the received master or slave pulses.

Accordingly, a principal object of the present invention is to automatically control the amplication of a loran receiver-indicator in accordance with the amplitude of the smaller of the received master or slave pulses.

Another object of the invention is to automatically control the gain of a loranreceiver-indicator similar to the manner in which it has been manually controlled heretofore by an operator.

A Afurther object is to provide a loran receiver-indicator with an automatic gain control system which automatically enables the receiver to amplify the smaller of the received master or slave pulses to a predetermined output level.

Still another object is to provide automatic increase of` the gain of a loran receiver-indicator to its maximum limit upon the loss of reception of either or both the master orl slave pulses.

In accordance with the present invention there is introduced an automatic gain control system responsive to `received first and second independent pulse voltages and producing an automatic gain control voltage according to the amplitude of the smaller of the first or second pulse voltages. The automatic gain control voltage is applied to a suitable control circuit for substantially suppressing variations in the amplitude of the smaller of the first or second pulse voltages. j

The above objects of and the brief introduction to the present invention will be more fully understood, and further objects and advantages will become apparent from a careful study of the following detailed description in connection with the drawing, wherein the single gure il'- lu'strates a combination block and schematic diagram of aV loran receiver-indicator employing the improved automatie gain control system of the present invention.

Those elements in the accompanying drawing fully corresponding to those in my aforesaid Patent 2,651,033 are identiied by the same reference numerals as employed therein.`

Referring to the single figure, loran A and B pulses of carrier-wave energy from remote master and slave stations are collected by antenna 11 and supplied to the input of superheterodyne receiver 12. Receiver 12 is identical to the receiver shown and described in the aforesaid Patent 2,651,033. amplified, detected, and supplied as positive A and `B pulses over lead 22 to `the cathode-ray tube indicator circuits 111, and over lead 23 to an input of the AFC circuits 116. An automatic gain control voltage from the AGC circuits of the present invention is supplied to the gain controlling electrodes of mixer 16, I. F. amplifiers 18, and the amplitude balance restorer 24 as will be explained more fully hereinafter. An amplitude balance control voltage also is supplied to receiver 12 as will be explained hereinafter.

The precision timing circuits of the loran receiverindicator comprise the oscillator and divider circuits 25, the square-wave generator 51, the A delay circuits '55, and the B delay circuits 60. yThese circuits are similar t0 those described and claimed in application S. N. 633,473, tiled December 7, 1945, in the name of Winslow Palmer, entitled Timing Apparatus and assigned to the same assignee of the present invention. They are identical with those shown and described in my aforesaid patent, and, therefore, will be described only briey in therpresent specification.

Oscillator cmd divider circuits The conventional oscillator and divider circuits of block 25- comprise a crystal-controlled oscillator operating at a frequency of kilocycles-per-second, and a cascade of ve frequency dividers, dividing the frequency of the oscillator output voltage in the steps of 5, 4, 5, l5, and 4 respectively, followed by a transient delay circuit. These frequency divider circuits supply the basic timing voltages for the loran receiver-indicator. The output voltage from the first frequency divider is supplied over lead 30 to one input of the B delay circuits 60, and over lead 31 to one of the inputs of the A delaycircuits 55. The output voltage from the third frequency divider is supplied over lead 35 to another input of the'A delay circuits 55, and over the lead 36 to a second input of the B delay circuits. The output voltage from the fourth frequency divider is supplied over lead 39 to a third input of the B delay circuits. The output voltage from the transient delay circuit is coupled overlead 50 to the input of the square-wave generator 51, and over lead 52 to the sweep circuits 106.

The basic pulse repetition rates used in loran are 331/3,

The received A and B pulses arer 3 letters H, L, and S. These pulse repetition rates are providedin the oscillator-divider circuits'ZS 'bythe'basic PRR switch S-8 coupled over lead 40 to the fifth frequency divider of the oscillator-divider circuits. This switch S-S controls the frequency division of the fifth frequency divider to provide a division of 3 for therate H, 4 for the rate L, and 5V for the rate S. VIn addition to the three basic pulse repetition rates H, L, and S,

Square-wave circuits The `positive output `pulse voltage on lead -50 from the oscillator and divider circuits is differentiatedat .the two inputs of an .Eccles-jordan circuit `used as a squarewave` generator 5110 Aproduce a `square-wave output voltage Whose frequency is equal ,to oneLhalf the repetition frequency of the differentiated triggering '.pulses. The frequency of this square-wave voltage corresponds ,to the pu'lse repetition frequency of the loran signals. The mark Yand space time intervals of the square-wave voltage are each equal to 20,000 microseconds for Ythe selected loran pulse repetition rate LO. The square-wave output voltage from generator Slis supplied to a pushpull cathode follower 53.

Y.Gathode follower 53 produces two square-wave output voltages,one invertediin phase with respect to the other, and one of .these square-wave voltages is supplied over lead V54 to the input of the A delay circuits 55 and to the B delay circuits 60. The other squarewave voltage is supplied over lead 56 tothe arm vof operations switch 843C. .Both of the square-wave voltages are supplied to ;the relay driver 132. The negative half-cycle of the square-wave .voltage on lead 54 energizes `the A delay circuits .55,.and this .Voltage is subsequently synchronized withrespect to the received loran signals so as to correspond `with the time interval during which the A pulses from tthe master station arrive at the receiver 12. The positive halfcycle `of the square-wave voltage on ylead 54 .energizes .the B delay .circuits 6.0, and corresponds to the time interval during which the B pulses from the slave station will arrive at the receiver.

A delay circuits .The `A delay circuits 55 comprise a pedestal delay circuit and a vpedestal synchronizer, as is -more fully described.in myvaforesaidV patent. The square-wave voltagezon lead54 is differentiated topproduce negative ;trigger-.pulses vcoincident with the trailing or fnegativefgoing edges `of the square-wave voltage, and these ynegative trigger pulses initiate lthe pedestal delay circuit. The voltage gon lead 35 from the third frequency divider is also differentiated and applied to the pedestal delay circuit to terminate the pedestal delay circuit by the first of the triggerpulses to arrive following the initiation of .the pedestal delay circuit. The output `from the pedestal delay circuit is a series of positive pulses of one-thousand microseconds duration and whose recurrence interval equals the recurrence interval of the squarewave voltage on lead 54.

'Both positive and negative voutput pulses from the pedestal delay circuit are applied toV the left-right switch S`7. The positivepulses are coupled .throughthe left position of switch S47 and through position l of-switch S-3F to the input of the lthird 'frequency divider over the'lead 47 to delay the triggering o'f the pedestal synchronizer.

third frequency divider by one more of its 200 microsecond 'input' pulses. 'This causes an increase in th recurrence interval of the `output pulses from the fifth divider which results in an increase in the recurrence interval of the sweep voltage applied to the cathode-ray tube indicator circuits 111. This increase in sweep recurrence interval causes the lreceived loran pulses to drift slowly across the face of the lcathode-ray .tube toward the left. Conversely, the negative pulses 'from the pedestal delay circuit are coupled to the right position of switch S-7 and -through position l1 -`of .switch S-SF and over lead 47 to the input of the third 4frequency divider in order to pretrgger -this divider -by `one less of its 200 microsecond input pulses. This reduction in recurrence interval results in alshorter sweep recurrence interval thereby causing the received loran pulses delineated upon the face of the cathode-ray tube to drift slowly across the face of the tube toward the right. When the left-right switch S7 is in its neutral position, there is no feedback of pulses and consequently there is very little if any drift of the delineated loran pulses.

The pedestal synchronizer is triggered by negative pulses derived from and coincident with the Vtrailing edges of the Vpositive output pulses from the pedestal delay circuit. The Vpedestal synchronizer is terminated by the first of the fifty microsecond negative trigger puis-es on lead 31 to arrive following the initiation of the The output from the pedestal synchronizer is a series of `positive pulses of approximately fifty microseconds duration and whose recurrence interval equals the recurrence interval of the square- .wave voltage on lead 54. The trailing edges of these output .pulses are delayed approximately one-thousand and .fifty microseconds from the trailing edges of the squareawave voltage on vlead S4, and the timing of the trailing edges of these ,output pulses is under the ac curate control of the pulses on lead 31 'from the firstv frequency divider.

These recurrent output pulses are coupled over lead 59 to the input of pedestal circuits 99.

B ydelay circuits and whose time delay with respect to the recurrent output ,pulses .from the A delay circuits S5 is adjustable by accurately known amounts indicated on a time difference counter 89.

circuits 60 and the recurrent pulses from the A delay circuits 55 in laddition -to the variable time delay introduced=byfthe B delay circuits.

The recurrent variably delayed output pulses on lead.

88 fromthe B delay lcircuits 60 are approximately 30 microseconds in duration, and are variable in time rela# tive to the leading'edges of the square-wavevoltage on lead 54 smoothly and unambiguously over the range of from 1,050 to almost 20,000 microseconds. Moreover;

the trailing edges ofthese variably delayed pulses vary in time relative to the trailing edges ofthe output'pulscs from'the A delay circuits 55 `on lead S9 continuously over the range of exactly 0 to This time delay difference is established with :an absolute ,accuracy better than l microsecond..

smoothly and almost '20,000A

The pedestal circuits- 991V comprise a pulse mixer' and a pedestalgenerator. Negative trigger pulses derived by differentiating thel trailingy edges otthe positive recurrent output pulses on leadl 59 `are combinedwith negati'veA trigger pulses derived by differentiating the trailingl edges of'the positive recurrent output pulses on lead 881' ink the pulsel mixer. Each of thesenegative trigger pulsesinitiate`4 tlie pedestal' generator, a monostable multivibrator, which` is terminatedfautomatically byA itsA own action. The pedestaligeneratorprovides a separatepositive andi a negative pulseoutput'voltage. These pedestal pulsesareof approximately 13300'mcroseconds duration for positionsl and 2* of" operations switch- S-SB, andA are' approximately 1-75 l microseconds duration forposi# tion 3- of' S-3Bf The positive pedestal output pulsesy are supplied over` lead 103L1 to" the-` arm' of operations switch S-3C`, and also to terminals 2*"andf3 of switch S-3A. Thev pedestal pulses initiated' by theV pulse voltage on lead 59produce theA pedestal; and'th'evariablydelayed* pedestal pulses initiated byl the pulse` Voltage on lead 88 prod'uce'the B pedestal. The square-wave voltage fromI the cathode followerl 53 appearing on lead` 56` isv cornbinedwith` the positive pedestal pulses on lead 103. The` negative pedestal pulses aresupplied over lead 105' to the sweep circuits 106 andalso to one' input of the' AFCcrcuitsll.

S'weep circuitsY TheI sweepA circuits-v 106- include a. gate generator, `al sweep generator for producing-a slow, mediurni. or fast' sweepspeed Voltage, and` aY sweepl restorer. Trigger pulses produced from. the. trailing-edgesj of therecurrent output voltage from the oscillator-divider circuits 25y on lead S2 initiate the sweep generator to produce the slow sweep-speedvoltage.A When the switch S3E is in position l, this slow sweep-speed voltage is supplied to one. input of` the cathode-rayptube indicator circuits 11.1.` The 'mediumv and-fast sweep-speed voltages are produced" when the operationsiswitch S-3E is yin the positi'onsgZ" and" 3", respectively and"y these sweep voltages are initiatedby the recurrent negative pedestal pulses. supplied' over lead'1`05lf The sweep generator produces a li'nean, medium' sweep-speed'voltage coincidentw-ith and for the duration of therecurrent negative. pedestal pulsesy when switch S-3E is set to position 2. Similarly,v the fast sweep-speed. voltage is coincident with'. and extends for tlietdration of the recurrent-negative pedestal pulses when, tli'e operationsrswitch. S-34 is set to position 3.'

Network 1'10" couplingb'asic PRR switch S-S with switcli S-3G` serves to maintain the amplitudes off the` three' sweep-speed` voltages of constant value for the three basicpulse repetitionv rates H, L, or Slt The` sweepV restorerv included within the. sweepcircuits 106. clamps the loweredges of' the three sweep-speedfvol'tages to areference voltagefleveln to insurethat tlie cathode-rayl trace on the face of the cathodeeray tube startsfrom the'V same pointV on the face for each of the three sweep voltages.

Catod-ray tube indicator circuits The: cathod'esray, tubeindicator circuits 111:. include a cathodeerayrtube, as horiziontalsweep; amplifier; avertical; amplifier; and: anaintensity: restorer. The sweep' vol-tages fromitne-sweep circuits106 are amplifiedlin.thehorizontalz sweepL-'arnpliferancl applied. to the horizontal; deflection plates: of; thezcathoderaytuhe 113i;y The vertical: amplifier:l amplifies thev composite voltage` consisting of: the; pedestaliipulsesron lead,v 103,., the square-wave voltage 2 on, leads` 56,-. and the; received loran.' Ai and: Bf pulsesf on lead,

223;. and supplies' these` voltages to Vthey vertical. deflection pla-tescof;the?cathodefraytube.-113* The pedestal` pulsesA om lead" 103 are. supplied through-1 positions..2 .and'3j` of switch ST3A to, the inputfof the` intensity restorer;v The restorer clamps; the upper: edges; of, thesefpositivei pedestal pulses toa fixed'. voltageflevel. corresponding,` to: normal intensity of the cathode-ray. trace on the faceof. the cathode-ray tube, and the negative portion of these :pedestal pulses, correspondingto the time intervals between sweeps, bias the control-grid` ofv the cathode-raytube, so as to blankthe cathode-ray beam.

A lltomalc" frequency' control circuits The automatic frequency control circuits 116,fare.si.m1 ilar to those describedandv claimed in Batent. 2,636,988 andare identical withthoseasshown andidescribed in my aforesaidv Patent.2,651,033. TheY AFC circuits 11-6,- inf clude an AFC delay circuit, any AFC. ampl-ier,.and.-an AFC synchronizer. Negative trigger pulses derived from the leading edges of thenegative AandB Vpedestal pulses on lead 105 initiate the AFC delay circuit. This circuit produces negative output pulses vofapproximatelTy microseconds duration, andzthese negative pulses. are..ap plied to a differentiatingcircuit at one vinput of theAFC synchronizer, and to the gain` synchronizer` 350 over l'ead'127. Y Y

`The dilerentiating,` circuit at one input of the AFC synchronizer produces firstV and secondlpositiveout'put trigger or sampling pulses from. the trailing edges ofthe negative 100 microsecond` pulses. The first positive .sampling pulses are delayed 10,0 microseconds from the lead? ing edges of. the negativeA pedestal pulses on lead. and the second positive sampling pulses are delayA l0() microseconds. from the.leading edges of'the negative- B pedestal pulses on'lead 105. p

Received' A and B pulses from'receiver. 12 are. sup# plied overleafd 23` to the AFC amplifier where they Vare further amplified andl supplied to. a differentiating circuit at another input of the .AFC synchronizer. l-TCswitch` S`4 coupledito' the AFC. amplifier places the AFCjci'r.- cuits. 116in operation. The output `of theAFClamplifer is grounded by the. left-righ?" switch. SLT to disable the operation of. the AFCduringtlie leftA or friglit'pos'L- tions to allow for proper operations of the drift ,circuits The AFC synchronizer produces first and secondi recurrent output pulses of. current. first pulses varies. according: to the relative time, position: or'coincidence between the applied differentiatedA pulses andVVV the applied first positive sampl'ingpulses from, tle differentiating circuit at the input of the AFC synchronizer: The amplitude ,of the second pulses of"current varies accordingi tol the relative time position or coir'iclr dence between the applied-'differentiated E4 pulses md the second'positive. sampling, pulses from the d'iierenti'atingcircuit. These rst and second'. outputpulses-ofcurrent are applied' to the armature 1'21 of relay 122:2 Tli'e relay is energized by the square-wave voltagefrcmI-tlie'; relay' driver 132 to separate the first 'output-. pulsesM of. current from theAFCl synchronizer from the secondgiout: put pulses of current.v The 'rst output pulses, varying, according to the relatve'time position of thedilerenti'ated A pulses withl respect to the applied ltirstpositive sampling pulses, are applied over lead 123 toa long. timeA constant filter 124 where they are integrated'fto produce the auto,-V matic frequency control voltage'. This AFC, Voltage. biases reactance tube 48A so as tomaintain theA frequency of the' 1:00'kilocycle-per-second oscillator inthe ,oscillator divider circuits 25 such that the first positive` sampling, pulses applied' to the AFC synchrouizer are coincident` with the differentiated A pulses.4

The magnitude of the controlH voltage on. lead` 49fis,v under the independent manual` control.` of the drift-.pcfl tentiorneter` 125 and,lef.tright switchiS-7f coupled tot the ffilter: 124-r Tlre:lefttrigh switch: S7: provides: two, fixed negative control `voltages of diferent'magnitudeseforr biasing-,reactance tubef 48', in. addition' tot supplying'feetl-ll backpulses to the; thirdl frequency divider.throughv switcllL The amplitudel of theVv S-3F as explained heretofore in connection with the A delay circuits 55. In the left position of switch S-7, one of these negative control voltages causes the delineated pulses on the face of the cathode-ray tube 113 to drift slowly across the face of the tube to the left, while in the right position of switch S-7 the other negative control voltage causes a drift of the delineated loran pulses to the right. The drift potentiometer 125 provides an adjustable negative control voltage from filter 124 for slowly drifting the delineated A and B pulses either to the right or to the left. These manual controls facilitate the alignment of the'received loran A and B pulses atop their respective A and B pedestals. The basic PRR switch S-S coupled to filter 124 through potentiometer 125 provides three separate time constants for the filter corresponding to the three basic pulse repetition rates H, L, or S.

Automatic amplitude balancing circuits lThe automatic amplitude balancing circuits now to be described are distinct from those described in my aforesaid Patent 2,651,033, and these circuits are more fully described and claimed in my application S. N. 403,771, filed concurrently herewith entitled Automatic Amplitude Balance Control System for Hyperbolic Navigation i Receivers, and assigned to the same assignee as the present invention. Recurrent negative 100 microsecond pulses are supplied from the AFC circuits 116 over lead 127 to a differentiating circuit at one input of a gain synchronizer 350. The differentiating circuit produces first and second four-diode switch type as shown in Fig. 10.10 on page 374 of the book Waveforms published by the McGraw- Hill Book Company, 1949.

v The gain synchronizer'produces first recurrent output pulses of currentwhose amplitude varies according to the relative time position or coincidence between the first positive sampling pulses and the loran A pulses, and produces second' recurrent output pulses of current whose amplitude varies according to the relative time position between the second positive sampling pulses and the loran Bpulses. Since the first positive sampling pulses have been made to occur coincident with the cross-over of the differentiated A pulse by action of the AFC system,` as taught in Patent 2,636,988, these particular positive sampling pulses occur at instants corresponding to the peak of the received loran A pulses. Accordingly, the output pulses of current from the gain synchronizer which result from the coincidence of the first positive sarnpling pulses and the A pulses vary according to the peak value of the A pulses.

In a similar manner, the second positive sampling pulses are brought into coincidence with the loran B pulses to produce output current pulses from the gain synchronizer which vary according to the peak value of the B pulses. Since the second positive sampling pulses are derived from the variably-delayed B pedestal pulses on lead 105, they are likewise variably-delayed pulses.`

1n order to bring the second positive sampling pulses into coincidence with the received loran B pulses, the time position of these positive pulses is varied under the control of coarse delay switch S-9 or the fine delay knob 96 of the B-relay circuits 60 in orderv to match the received loran A andBpulse's on the face ofthe cathode-ray tube 113 as in the normal operating procedure. When the A and B pulses are properly matched on the face of the cathoderay tube 113, the second postive sampling pulses are coincident with the peak value of the received loran B pulses.

The first and second recurrent output pulses from the gain synchronizer 350 are coupled to the armature or movable contact 130 of relay 131. The winding of relay 131 is energized by the square-wave voltage from the relay driver 132. VThe armature 130 of relay 131 vibrates in synchronism with the square-wave voltage to separate the first recurrent output pulses from the second recurrent output pulses. The first output pulses of current varying according to the amplitude of the A pulses are supplied to a condenser 352, and the second output pulses of current varying according to the amplitude of the received B pulses are supplied to a condenser 353. The condenser 352 is charged to a potential varying according to the value of the first current pulses, and the condenser 353 is charged to a potential varying according to the value of the second current pulses. The armature' 130 of relay 131 alternates between the charged potential on condenser 352 and the charged potential on condenser 353 at the frequency of the square-wave voltage supplied` to the relay 131. This alternating voltage on armature 130 is the automatic amplitude balance control voltage, as will be more fully explained in the aforesaid copending application S. N. 403,771, and this AABC voltage is supplied through cathode follower 354 and amplifier 355 to the amplitude balance restorer 24 in receiver 12.

Control box 135 includes an automatic balance control on-off switch 136, a manual gain control 137, anda manual amplitude balance control 138, as explained in my aforesaid Patent No. 2,651,033. When the switch 136 is in the off position, the control box supplies manually adjustable control voltages across each condenser 352 and 353. The manual gain control 137 raises and lowers the applied control voltages together, and the manual amplitude balance control 138 raises the voltage supplied to one condenser while lowering the voltage supplied to the other condenser.

Automatic gain control circuits The automatic gain control circuits of the present invention include a double-triode amplifier and a doublediode selector situated within boX 356. These circuits within box 356 are responsive to the potentials across the condensers 352 and 353, as supplied over leads 357 and 358, to produce an automatic gain control voltage in a manner now to be described. The voltage across condenser 352 is amplified in the first triode section of double-triode amplifier tube 359, and similarly, the voltage across condenser 353 is amplified in the second triode section of tube 359. The magnitude of the voltage on anode 360 of the first triode section varies inversely with respect to the peak amplitude of the received loran A pulses, and the magnitude of the voltage on anode 361 of the second triode section varies inversely with respect to the peak amplitude of the received loran B pulses. Anodes 362 and 363 of the double-diode tube 364 are conpled respectively to the anodes 360 and 361 of tube 359.

The cathodes 365 and 366 of double-diode 364 are coupled together and to the upper terminal of a voltage divider composed of resistors 367 and 368. A negative voltage is supplied to the lower terminal of resistor 368.

The voltage` on the inter-coupled cathodes 365 and 366 is determined by the smaller of the output voltages onthe anodes 360 and 361 of the tube 359 in the following .mannen For the condition where the amplitude of the received A pulses is smaller than the amplitude of the received B pulses, the magnitude of the voltage across condenser 352 is smaller than the voltage acrossv condenser 353. Accordingly, the voltage at the intercoupled anodes 360, 362 vwill be larger (more positive) than the voltage on the inter-coupled anodes 361, 363. As a result, the first diode section of tube 364 will conduct raising the voltage on its cathode 365 to a value very` nearly equal to the voltage on its anode 362. Since the.

voltagerapplied to anode `362Y isfmore positive than the voltageapplied to. anode 363, `thevoltage on the intercoupled cathodes-is likewise morepositive than the applied voltage on anode 363. Accordingly, the second diode tube Vwill not conduct, and the cathode voltage is determined by and very nearly equal to the voltage on the inter-coupled anodes 360 land 362. The voltage divider sets the voltage level at the junction of resistors 367 and 368 ata suitable negative level, and the control voltage at this junctionA is the automatic gaincontrol voltage. This AGC voltage is supplied to the gain controlling electrodesof the mixer'16, I. F. vamplifiers 18, and the amplitude 'balance 1restorer 24 in receiver 12. As the voltage on the inter-coupled anodes 360, 362 becomes more positive due toa decrease in the amplitude of the received A pulses, the AGC voltage at the junction of resistors 367 and 368 will Vbecome less negative which will cause an increase in receiver gain, thereby tending to suppress variations in thestrength of the received output loranA pulses. For the condition where the amplitude of the received B pulses issmallergthan the amplitude of the received A pulsespthe magnitude `of the voltage across condenser 353 is 4smaller than the Vvoltage across condenser 352. Accordingly, the voltage on the inter-coupled anodes 361 and 363 will be more' positive than'the voltage on the inter-coupled anodes 360, 362. The second section of diode 364 will now conduct, and the voltage on the intercoupled cathodesV 365 and 366 is determined by the voltage on the anode 363. The AGC voltage will vary the gain of receiverlZ in a manner which tends to suppress variations in the amplitude of thereceived B pulses.

Theloran receiver-indicator with, the improved automatic` gain controlsystem of thepresent invention is adjusted by an operator to obtain useful navigationalinformation in an identical manner as explained` in my aforesaid Patent 2,651,033 under the section entitled Operation of Improved Loran Receiver-Indicator.

The AGC system 'of the present invention is not limited solely to manually operated loran receiver-indicators .but may; be employed inautomatic tracking loran'receiveritldatofs; f the ,typedcscribed and claimed in' pending application S. N. 267,347, now Patent 2,697,219, filed on January 21, 1952, in the name of Roger B. Williams, Jr., entitled Automatic Time Difference Measuring Circuits and assigned to the same assignee as the present invention.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is: l

1. In a radio navigation receiver responsive to recurrent A pulses transmitted from a master station and to recurrent B pulses transmitted from a slave station, wherein the strength of the received recurrent A pulses may be different from the strength of the received recurrent B pulses, said receiver including an electrically controllable variable gain amplifier: an automatic gain control system comprising means coupled to the output of the receiver for switching the received A pulses into a first channel and the received B pulses into a second channel, first means coupled tothe output of said switching means and responsive to the recurrent A pulses in said first channel for producing a first output voltage version varying according to the strength of said A pulses, second means coupled to the output of said switching means and responsive to the recurrent B pulses in said second channel for producing a second output voltage version varying according to the strength of said B pulses, means coupled to the output of said first and second producing means and responsive to said first and second voltage versions forproducing an output control voltage varying according to the strength of the smaller of said first and second voltage versions, and means coupling said control voltage to the gain control circuit of said variable gain amplifier for substantially suppressing variations in the strength of the smaller of said received output A and B pulses. 1

2. In a radio navigation` receiver responsive to recurrent A pulses transmitted from a master station and to recurrent B pulses transmitted from a slave station, wherein the strength of the received recurrent A pulses may be different from the strength of the received recurrent B pulses, said receiver including an electrically ,controllable variable gain amplifier: an automatic gain control system comprising means coupled to the output of said receiver for producing a first voltage version varying according to the strength of the received A pulses and a second voltage version varying according to the strength of the received B pulses, means coupled to the output of said producing means and responsive thereto for producing an output control voltage varying according to the strength of the smaller of said received A and B pulses, and means coupling said control voltage to the gain control circuit of said variable gain amplifier for substantially suppressing variations in the strength of the smaller of the received output A and B pulses.

3. In a hyperbolic navigation receiver receptive to master pulses and slave pulses from distant transmitters wherein the strength of the received master pulses may be different from the strength of the received slave pulses, said receiver including an electrically controllable variable gain amplifier: an automatic gain` control system comprising means coupled to the output of said receiver and producing a first voltage version varying according to the strength of said received master pulses and a second voltage version varying according to the strength of said received slave pulses, means coupled to the output of said producing means and responsive to said first and second voltage versions for producing an output control voltage whose` strength varies according to the strength of the smaller of said first and second voltage versions and independently of the strength of the greater of said versions, and means intercoupling said responsive means and the gain control circuit of said variable gain amplifier for varying the amplification of said receiver in accordance with said control voltage.

4. An automatic gain control system for a hyperbolic navigation receiver responsive to recurrent A pulses transmitted from a master station and to recurrent B pulses transmitted from a slave station, comprising means coupled to the output of said receiver for switching the received A pulses into a first channel and the received B pulses into a second channel, first means coupled to the output of said switching means and responsive thereto for producing a first control voltage varying according to the peak value of the received A pulses, second means coupled to the output of said switching means and responsive thereto for producing a second control voltage varying according to the peak value of said received B pulses, selective means coupled to the output of -said first and second producing means for selecting the smaller of said first and second control voltages, and means coupling the selected control voltage to said receiver for substantially suppressing variations in the amplitude of the smaller of the received output A and B pulses.

5. An automatic gain control system comprising variable gain amplifier means receiving first and second independent pulses to be amplified, means coupled to the output of said amplifier for separating the amplified first pulses from the amplified second pulses, means coupled to the output of said separating means for producing an output control voltage varying according to the strength of the smaller of said first and second amplified pulses, and means coupling said control voltage to the gain control circuit of said variable gain amplifier means for substan- -l tially suppressing -variations Iin the strength of the smaller of lsaid ftirst an'dfsecond lamplied pulses-f Y `6'. lll'fariable` gainl'radio receivingappa-ratus for-receiving rstand second recurrentsignal Wavevtrainsysaid receiving apparatus comprising a-'controllabletransmission 'circuit transmitting said received first and second lwave trains, and means responsive to said received irst-and second wave trains for Aselectively varying the average transmission response of'said `controllable "transmission circuit inversely according .to the strength of :the t weakest ofsaid iirst and :second recurrent wave trains.

1'7. An automatic `control ycircuit 'comprising means alternately producing la rst'pulse duringa-ii-rst-.tirne1interval land fa second pulse during ta l second time interval, said rneansxinclu'dingl an Yelectrically controllable transmission circuit 'for transmitting said Aiirst and l second pulses, means A-coupled `-to -the --output -ofv said producing means 4andresponsive-to-said iirst andgsecondpulses for producing an'output `controlvoltage varying according to the strength of-thelsmaller o'f theiirst-and second-output i pulses, andmeans .couplings-said control voltage to-said electrically f controllable ltransmission :circuit yfory substantially :suppressingyariation of thestrength offthe smaller of :saidfrst and second output'pulses.

7S. 'An .automatic control Vcircuit vcomprising =means. s ins cluding an electrically controllable transmissionfcircuin said meansbeing-adapted lto lreceive fappliedvoltage ywaves consisting-of recurrent first pulses occurring lduring iirst time intervals Y.and recurrent second pulses-*occurring during-second time intervals, -means :coupled toftheout-putof said transmission circuitmeans andresponsivetothe output recurrent first and :second-pulses -for producing an output control 4voltage varying laccording to thestrength of the Asmalleroisaid voutput-recurrentiiirst land second pulses, and means coupling said control voltageto said electrically Acontrollable .transmission .circuit means for substantially suppressingvariations in lthe strength -o'f Vthe smaller of saidoutput recurrent v.first 'and second pulses.

9. In a radio navigation receiver responsive to-recurrent A pulses transmitted 'froma master station and to recurrent B pulses transmitted -from a slave station, wherein fthe strength of the .received v'recurrent A pulses may be ydifferent from V'the strength of the received -recurrent LB pulses, said receiverincluding'an electrically-controllablelvariable :gain amplier: 'an automatic'gain control system comprising lmeans ycoupled to the output of saidreceiverffor producing a iirst'cont'rol voltagevarying according 1tothe strengthof the received A pulses and producing a-second control voltage varyingaccordingto the` strength of the received `B pulses, Vselective -means coupled to theoutput of said producing -means `and -re' sponsive` yto said rlirst and second control 'voltages for selecting one ofsaid first and'secondcontrol'voltages, and? means coupling saidselected control 'voltagetothe gainV control circuito'ffsaid variable gain arnpliiierffor-substantially-fsuppressing variations in the amplitude -o'f fone of the received output -Afand rB pulses. 1 '10. The automatic gaincontrol ysystemas defined in' clairn9, wherein said means coupled tothe-outputofsaidv receiver for producingsaid irst control voltage andfsaid second control voltage includes first and second venergy storage means responsive respectively tothe :.pealcvalues of thefreceived A and B pulses, and-whereinisaid selective means coupled to the output of said producing means select thesmaller of said first and second controllvoltages. -1l. The automatic gain control system as `deiinedin claim 10, wherein said selective means-coupledto the-output of said producing Ymeans includes first `and second intercoupled rectiiiers responsive respectively'to the magnitudes of-said lirst and second controll voltages'.

.12.'The automaticgain control systemasdeiinedin t claim l() wherein `said fmeanscoupled to -^the-output-of saidreceiverffor producing said iirst^control=voltage and said second controlfvoltage further'includesrelayswitchingmeans'for coupling the -outputofsai'd receiver tosaid iirst `energy storage means during reception of -said A pulses and forcoupling theoutput of said receiverto said second energy Ystorage means during reception of :said'B pulses. f f

Pierce I une 10, 1952 Frantz -'Sept. 1, v1953 

